Cardiac pacers are used for regulating heart beats in patients who suffer from a failure of the natural regulatory system. The cardiac pacer includes one or more electrodes carried on leads which are enclosed in an insulating sheath, with the one or more electrodes communicating with the exterior so that they can be in contact with the heart.
Typically, cardiac pacers are designed for entry through a vein to the ventricle of the heart (pervenous) or, alternatively, the cardiac pacer may be sutured onto the ventricle of the heart (myocardial).
Insulated sheaths of cardiac pacers which are currently available may be made of silicone elastomer, polyurethane, or polytetrafluoroethylene (Teflon). Polyurethanes are particularly growing in favor for use as insulation for an electrical lead because of its strength and flexibility. This for example makes possible the placement of two pervenous leads into one vein since the polyurethane insulation can be thinner because of its higher strength. It has been particularly desirable to use harder grades of polyurethane as an insulating sheath (a Shore D hardness of about 55 or more).
However, this relatively stiff insulation, while having advantage due to its high biological stability, is considered by many to be too stiff for safe use within the atrium or ventricle of the heart, even when thin-walled insulation is used so that the lead has a diameter of only about 2 millimeters. There is a concern that while it is desired for portions of the cardiac pacer lead which are remote from the heart to have stiff insulation, there is a risk that stiff insulation in the immediate vicinity of the heart can damage the heart tissue as the heart beats.
Accordingly, while it would be desirable to use stiff polyurethane formulations in a cardiac pacer lead for advantages in placement of the lead in blood vessels and the like, there is a major potential disadvantage in the use of a cardiac pacer having stiff insulation in that it may damage the heart.
By this invention it becomes possible to make use of the desirable stiff insulating material in electrical leads and particularly cardiac pacer leads to achieve the advantages resulting therefrom, while at the same time providing protection to the heart from damage due to the stiff insulation.